Their main competitors were a closely related group of reptiles called the crurotarsans, from which modern crocodiles and alligators descended.

Crurotarsans and dinosaurs coexisted for about 30 million years. But about 200 million years ago, Earth suffered a mass extinction, possibly caused by rapid global warming. (Learn about global warming today.)

Most crurotarsans disappeared, leaving dinosaurs to inherit Earth.

"I think the answer is just blind luck," said Stephen Brusatte, a Ph.D. candidate at Columbia University in New York and paleontologist at the American Museum of Natural History.

"There was this big extinction event 200 million years ago, and for some reason it hit the crurotarsans very hard.

"Crurotarsans survived the extinction, but they were truncated basically just to crocodiles."

Brusatte co-authored the new study, which appears tomorrow in the journal Science.

Would-Be Dominators

Some scientists have argued that some feature or characteristic of the dinosaurs allowed them to beat their competition and survive the mass extinction.